PMDC motors employ magnetized materials to supply pole flux. Some materials employed for use in PMDC motors may be found among several classes of permanent-magnet materials, including alnicos, neodymium-iron-borons, ceramics (ferrites), and rare-earth materials. Because a permanent-magnet (PM) is used, the flux per pole, .PHI., is not adjustable in PMDC motors. The static torque which can be applied to an unexcited motor without causing continuous rotation is termed the detent torque. The latter torque arises in PM motors owing to the continual presence of a magnetic field even in the absence of applied current (excitation) to the armature winding.
The position at which a PM rotor comes to rest without excitation at no-load is termed the detent position. The detent position is the location where detent torque equals zero.
In PMDC motors, the armature circuit includes, in part, an armature winding about a portion of the stator core. The armature winding is sometimes referred to as a stator coil or stator winding in brushless PMDC motors, the armature winding receives a current which produces a "revolving" flux in the air gap between the stator and rotor. This revolving flux causes the PM rotor to revolve with respect to the stator for producing torque. The torque produced when the motor is excited is a combination of mutual and detent torques. Mutual torque arises from the forces of attraction and repulsion between the poles of the PM and the opposite electromagnet poles of the excited stator coil.
As is known, motors may be single-phase or polyphase. In single-phase PMDC motors, only one set of armature windings is employed. Thus, one or more "coils" in series may be used for the armature winding, while retaining the single-phase characteristic.
A problem associated with single-phase PM motors is starting. Single-phase PM motors detent at detent torque nulls (locations where no detent torque exists). If these detent torque nulls coincide with mutual torque nulls, then there is no torque for starting the motor when current is applied. In order to start a single-phase PMDC motor, current must be delivered to the motor, and the PM rotor must be at a position where useful torque is delivered to the motor. As applied to spindle ("spinning") motors for hard disc drives, single-phase PMDC motors must be started prior to obtaining constant rotation.
French Patent Nos. 772.324 and 1.033.643 show clock movements. Specifically, in Patent No. 1.033.643 an external stator stepper motor is shown having asymmetrical air gaps.